The xerographic art has recognized that one of the most desirable devices to fuse dry particulate toner is a hot roll fuser. In a hot roll fuser a sheet of plain paper carries a resin-based toner image, and the toner directly contacts the hot surface of a heated roll, as this hot roll cooperates in pressure contact with a second (backup) roll to form a fusing nip.
In some instances, a portion of the toner image, now tackified by heat and pressure as it passes through the fusing nip, sticks or offsets to the surface of the hot roll. Later, and undesirable, this toner may partially transfer to the next sheet of paper as it passes through the fusing nip. This offset toner can also contribute to the occurrence of fuser wraps, i.e., a failure condition wherein the sheet's leading edge sticks to the hot roll on the downstream side of the fusing nip.
If such a failure occurs, the sheet wraps about the hot roll, and must be manually removed.
Various have been devised to assist in release of the sheet's leading edge from the hot roll.
U.S. Pat. No. 3,955,813 is an example of a pneumatic peeler bar for this purpose. In addition, the use of a soft fusing roll and a hard backup roll provides a fusing nip whose geometry tends to cause the sheet's leading edge to release from the soft hot roll. Aforementioned U.S. Pat. No. 3,955,813 is of this type.
These and other hot roll fusing problems led to the development of so-called wet release hot roll fusers wherein the hot roll cooperates with a relatively complex mechanism that functions to continuously supply a release oil, for example silicone oil, to the toner-engaging surface of the hot roll. These fusers are more expensive, require periodic maintenance of the oil applying mechanism, and require periodic addition of oil by the user. In addition, there may be a propensity of the hot roll's elastomeric surface material to swell as oil is absorbed.
These and other disadvantages of wet-release hot roll fusers led to refinement of the hot roll's elastomeric surface, and to the eventual commercial development of the so-called dry release hot roll fuser. U.S. Pat. No. 3,848,305 is exemplary of such a fuser. The hot roll of this patent comprises a cylindrical metal core whose major outer surface length is covered by a resilient RTV-60 silicone elastomeric material, wherein the content of volatile cyclic siloxane components have been reduced in a specified manner. RTV-60 is a brand of silicone rubber by General Electric Company. U.S. Pat. No. 4,188,423 purports to not only remove cyclic siloxanes from such a roll, but to also process the roll so as to remove other low molecular weight siloxanes, polysiloxanes and/or other impurities.
In addition to the use of RTV silicone rubber, as above mentioned, the prior art has recognized the use of LTV (low temperature vulcanization) silicone rubber, and HTV (high temperature vulcanization) silicone rubber.
A family of prior art patents also exists where dry release is said to be accomplished by the use of silicone rubber into which a catalytic agent is dispersed. This agent, in the presence of water or moisture, promotes the degradation of the silicone rubber at elevated temperatures, such that a degradation product is produced which itself is a release material for the hot, tackified toner. U.S. Pat. Nos. 3,976,814; 3,997,691; 4,000,339; 4,054,410; 4,075,390; 4,107,389 and 4,126,722 are of this type.
The dry release hot roll fuser of U.S. Pat. No. 4,019,024 provides a silicon rubber which is impregnated with silicon oil, the oil being in the form of so-called free oil.
A more complex rubber formulation is taught by U.S. Pat. No. 4,074,001 wherein RTV silicone rubber comprises a mixture of two diorganopolysiloxanes, one of which is terminated at the chain ends with high viscosity silanol groups, and the other of which is terminated at the chain ends with low viscosity trialkyl silyl groups. An alkoxysilane or a partial hydrolysis-condensate is the crosslinking agent, and metal salt of an organic acid is the reaction catalyst. Calcium carbonate, iron oxide and titanium dioxide are fillers, but substantially no siliceous fillers are used since it is said that siliceous fillers are harmful in the maintenance of the release property of the cured silicone rubber composition. U.S. Pat. Nos. 3,795,033 and 3,669,707, which themselves suggest the use of various silicone elastomers as a hot roll fuser release coating, support this teaching-away from the use of high surface energy fillers such as silica.